Organically crosslinked polyvinyl alcohol copolymeric gels for use under harsh reservoir conditions

ABSTRACT

A gel-forming composition capable of plugging highly permeable zones in a subterranean formation. The composition comprises water, a water-dispersible polyvinyl alcohol copolymer and a crosslinking agent which is a mixture of an aldehyde and a phenolic component in an amount effective to cause gelation of the aqueous solution of the copolymer. The resultant gel is exceptionally thermally stable and, therefore, can be used as an effective profile control agent in all enhanced oil recovery operations, including steam flooding. Also provided is a process for recovering oil from a subterranean oil-bearing formation.

FIELD OF THE INVENTION

This invention relates to novel crosslinked polyvinyl alcohol copolymer gels and to the use thereof in controlling the permeability of subterranean oil-bearing formations.

BACKGROUND OF THE INVENTION

In the production of oil from subterranean formations, it is usually possible to recover only a small fraction of the total oil present in the formation by so-called primary recovery methods which utilize only the natural forces present in the reservoir. To recover oil beyond that produced by primary methods, a variety of supplemental production techniques have been employed. In these supplemental techniques, commonly referred to as secondary or tertiary recovery operations, a fluid is introduced into the oil-bearing formation in order to displace oil to a production system comprising one or more production wells. The displacing or "drive" fluid may be an aqueous liquid such as brine or fresh water, a gas such as carbon dioxide, steam or dense-phase carbon dioxide, an oil-miscible liquid such as butane, or an oil and water-miscible liquid such as an alcohol. Often, the most cost-effective and desirable secondary recovery methods involve the injection of steam. In practice, a number of injection and production wells will be used in a given field arranged in conventional patterns such as a line drive, a five spot or inverted five spot, or a seven spot or inverted seven spot.

In the use of the various flooding techniques, it has become a common expedient to add various polymeric thickening agents to the drive fluid to increase its viscosity to a point where it approaches that of the oil which is desired to be displaced, thus improving the displacement of oil from the formation. The polymers used for this purpose are often said to be used for "mobility" control.

Another problem encountered is that certain injected drive fluids may be much lighter than the reservoir fluids and thus separate by gravity, rising toward the top of the flowing region and resulting in the bypassing of the lower regions. This phenomena is known as gravity override.

Also encountered in the use of the various flooding techniques is a situation caused by the fact that different regions or strata often have different permeabilities. When this situation is encountered, the drive fluid may preferentially enter regions of higher permeability due to their lower resistance to flow rather than the regions of low permeability where significant volumes of oil often reside.

It therefore is often desirable to plug the regions of high permeability, or "thief" zones, either partly or entirely, so as to divert the drive fluid into regions of lower permeability. The mechanical isolation of these thief zones has been tried but vertical communication among reservoir strata often renders this method ineffective. Physical plugging of the high permeability regions by cements and solid slurries has also been tried with varying degress of success; however, these techniques have the drawback that still-productive sites may be permanently closed.

As a result of these earlier efforts, the desirability of designing a system capable of sealing off the most permeable layers so that the drive fluid would be diverted to the underswept, "tighter" regions of the reservoir, became evident. This led to the use of oil/water emulsions, as well as gels and polymers for controlling the permeability of the formations. This process is frequently referred to as "flood conformance" or "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed include oil/water emulsions, gels, e.g., lignosulfate gels and polymeric gels, with polymeric gels being the most extensively applied in recent years.

Among the polymers so far examined for improving flood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides, both in their normal, non-crosslinked form, as well as in the form of metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines, low pH and high temperature. To overcome these problems and to achieve deeper polymer penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ.

Another group of polymeric thickeners which has received considerable attention for use in improving flooding are polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose a process for mobility control by the use of polysaccharides.

U.S. Pat. Nos. 3,741,307, 4,009,755 and 4,069,869 disclose the use of polysaccharides in the control of reservoir permeability. U.S. Pat. No. 4,413,680 describes the use of crosslinked polysaccharides for selective permeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man-made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides is also disclosed in U.S. Pat. No. 3,810,882.

Another type of polysaccharide which has been experimented with in the area of profile control is the non-xanthan, heteropolysaccharide S-130. S-130 belongs to the group of welan gums and is produced by fermentation with a microorganism of the genus Alcaligenes. Another welan gum heteropolysaccharide, known as S-194, is also produced by fermentation with a microorganism of the genus Alcaligenes. A notable characteristic of the heteropolysaccharide S-130 is that it develops a high viscosity in saline waters. This is particularly so in brines which contain divalent cations such as Ca²⁺ and Mg²⁺ or monovalent cations such as Na⁺ and K⁺. U.S. Pat. No. 4,658,898 discloses the use of welan gum S-130 in saline waters. Crosslinking with trivalent cations, such as chromium, aluminum, zirconium and iron is also disclosed. Additionally, crosslinking with organic compounds containing at least two positively charged nitrogen atoms is disclosed in U.S. Pat. No. 4,658,898; while Ser. No. 283,399, filed on Dec. 12, 1988, discloses welan gums crosslinked with phenolic resins or mixtures of phenols and aldehydes.

The use of various block copolymers for mobility control in waterflooding operations is described in U.S. Pat. Nos. 4,110,232, 4,120,801 and 4,222,881. Chung et al., U.S. Pat. No. 4,653,585, disclose the use of block copolymers, which may be crosslinked with polyvalent metal ions, for use as permeability control agents in enhanced oil recovery applications.

While a number of the different compositions discussed have been proposed for permeability control, some of these compositions may be unsuitable for use as permeability control agents under certain circumstances. For example, the polymers of Chung et al., may not be effectively crosslinked with polyvalent metal ions under all conditions encountered in the enhanced oil recovery applications, e.g., in acidic conditions commonly found in carbon dioxide (CO₂) flooding operations. Polyacrylamides display instability in the presence of high brine concentration at temperatures over 70° C. Xanthan gums are very brine tolerant but display thermal instability, even at temperatures below 60° C. Still, other polymers are unsuitable as permeability control agents when used in conjunction with steam flooding operations. This is due to the fact that they lose their structural integrity (i.e., they undergo "syneresis") at the high temperatures generated during such operations.

Syneresis is the contraction or shrinking of a gel so that liquid is exuded at the gel surface. For example, a gel said to exhibit 20% syneresis would take up 80% of its original volume, with the remaining 20% being expelled water. Although the exact mechanism responsible for the syneresis of such gel-forming compositions is not fully understood, it is believed to result from the over-crosslinking of the polymeric material with time. While it is not yet known what an acceptable level of syneresis might be for profile control gels, it is believed that to minimize syneresis would enhance the effectiveness of such gels.

In view, therefore, of the severe conditions which include both high brine concentrations and elevated temperatures, there is a need for brine tolerant, thermally- stable materials suitable for high temperature wells and steam flooded wells. This has led to the development of the so-called hostile environment polymers, such as those marketed by the Phillips Petroleum Company of Bartlesville, OK and the Hoechst Celanese Corporation of Somerville, N.J.

It has been known to use polyvinyl alcohol (PVA) metal-crosslinked gels, as described in U.S. Pat. No. 3,762,476 to Gall, in order to correct subterranean formation permeability. See also U.S. Pat. No. 4,039,029 to Gall and U.S. Pat. No. 4,018,286 to Gall, et al. Unfortunately, polyvinyl alcohol has a drawback in that it very poor cold water solubility, and has to be dispersed in cold water as a suspension followed by heating to a temperature of, for example, 90°-95° C. for dissolution. Furthermore, polyvinyl alcohol also has the limitation that the choice of possible crosslinking metal components is limited, titanium (IV) probably being the only practical choice. Other commonly used metals such as aluminum, chromium, and zirconium are not effective to crosslink polyvinyl alcohol. Furthermore, polyvinyl alcohol gels crosslinked with titanium are not brine stable at high temperatures. In particular, syneresis can occur.

Polyvinyl alcohol can also be crosslinked covalently with polyaldehydes, as described in the GB Patent Application No. 2,145,420 A. of Marocco et al. The gelation described in the Marrocco et al. disclosure, however, requires an acidic environment, creating a need for controlling the pH of the reservoir to facilitate the process. Needless to say, control of the reservoir environment at a particular pH can be a very difficult task. Regardless of the alternative crosslinking process described by Marrocco et al., the polyvinyl alcohol solution preparation problems remain and the gel stability in strong brine concentrations is uncertain.

Ser No. 092,274, filed on Sept. 4, 1987, the inventors of which are also the inventors of the present invention, discloses crosslinked polymers obtained by crosslinking polyvinyl alcohol or a derivative thereof with a crosslinking agent which is a mixture of a phenolic component and an aldehyde or a mixture of a naphtholic component and an aldehyde. While such crosslinked polymers are stable even at the high underground formation temperatures encountered during steam flooding oil recovery operations, the polyvinyl alcohol solution preparation problems persist for many of the compositions disclosed. Ser. No. 092,274 is hereby incorporated by reference in its entirety.

Polyvinyl alcohol is not a very water soluble polymer. This is because its compact structure and many hydrogen bonds lead to high crystal energy [See, T. H. Kwei in "Macromolecules, An Introduction to Polymer Science", (F. A. Bovery and F. H. Winslow, ed.) Academic Press, N.Y., 1979, p. 273]. Disrupting the crystal packing of polyvinyl alcohol, such as by copolymerizing with a suitable monomer can greatly improve the resultant copolymer's solubility in water.

U.S. Pat. No. 4,678,032, the inventor of which is a co-inventor of the present invention, teaches multivalent transition metal crosslinked polyvinyl alcohol copolymeric gels useful for profile control under severe reservoir conditions. The copolymers taught are selected from the groups consisting of poly (vinylalcohol-co-vinyl-carboxyl and poly (vinylalcohol-co-vinylether). U.S. Pat. No. 4,678,032 is hereby incorporated by reference in its entirety.

Despite these developments, a need still exists for permeability control agents which are easy to prepare and are compatible with the harsh conditions encountered in steam flooding enhanced oil recovery operations.

It is, therefore, an object of the present invention to provide a polymer gel-forming composition for injection in a subterranean reservoir which has good thermal stability at high brine concentration.

It is another object of the present invention to provide a polymer gel which can be used effectively as a permeability control agent under the extreme temperature conditions encountered in the steam flooding of underground formations.

It is a further object of the present invention to provide an aqueous crosslinked polyvinyl alcohol copolymer gel which is easily prepared.

Other objects and the several advantages of the present invention will become apparent to those skilled in the art upon a reading of the specification and the claims appended thereto.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an aqueous-based polymeric composition capable of forming a stable gel under harsh subterranean formation conditions which is comprised of water, a water-dispersible copolymer of polyvinyl alcohol and a crosslinking agent which is a mixture of a phenolic component and an aldehyde. The gel-forming composition is an effective permeability control agent which is stable even when conditions of high salinity and high underground formation temperatures are encountered. The gel-forming composition of the present invention finds particular utility in steam flooding operations.

Description of the Preferred Embodiments

Any water-dispersible or water-soluble polyvinyl alcohol copolymer capable of forming stable aqueous gels in the presence of an organic crosslinking agent is contemplated for use in the practice of the present invention. Preferred copolymers are those which are water-soluble, with cold-water solubility being a particularly preferred characteristic of the copolymers described herein.

The water-soluble copolymers of polyvinyl alcohol which are preferred in the practice of the present invention are selected from the group consisting of (1) copolymers of vinyl alcohol and vinyl alkylsulfonate ethers, having the following structure: ##STR1## wherein: R is an alkyl or aryl group having no more than 10 carbon atoms,

M⁺ is Na⁺, K⁺ or other counter ions, and

a and b are mole fractions of each co-monomer unit such that a+b=1, and a>o and b>o; and (2) copolymers of viny alcohol and vinylacrylamido ether, having the following structure: ##STR2## wherein: R is an alkyl or aryl group having no more than 10 carbon atoms; and

a and b are mole fractions of each co-monomer unit such that a+b=1, and a>o and b>o.

Particularly preferred copolymers include poly(vinyl alcohol-vinyl propanesulfonate ether) [P(VA-VPSE)] and poly (vinyl alcohol-co-vinylacrylamido ether) [P(VA-VAE)], the former being of the structure of equation (1), the latter being of the form of equation (2). In general, copolymers in which the comonomer is present in an amount of 1-30 mole % are preferred for use in profile control applications, with comonomer contents of 1-10 mole % particularly preferred.

Any commercially-available polyvinyl alcohol may be used to form the copolymeric materials required for the practice of the present invention. Polyvinyl alcohol (PVA) is a well known polymer prepared by replacing acetate groups of polyvinyl acetates with hydroxyl groups. The alcoholysis reaction proceeds most rapidly in a mixture of methanol and methyl acetate in the presence of catalytic amounts of alkali or mineral acids. Polyvinyl alcohol and the synthesis thereof are described in greater detail by D. L. Cincera in Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Third Edition, John Wiley & Sons, New York, 1983, Volume 23, pages 848-865, the entire contents of which are incorporated by reference herein. PVA may be obtained from the Aldrich Chemical Company of Milwaukee, Wis. and other sources.

As described in U.S. Pat. No. 4,458,052, which is incorporated by reference in its entirety, the incorporation of from about 5 to 40 mole % of alkyl sulfonate groups, e.g., C₂ -C₆ alkyl sulfonate groups, particularly 3-propane sulfonate groups, onto the alcohol oxygens of polyvinyl alcohols imparts solubility to the resulting copolymers at high concentrations of salt and divalent ions. When such groups are introduced into the polyvinyl alcohol by direct copolymerization, the copolymers of equation (1) are produced. Such copolymers can be produced by well-known emulsion copolymerization techniques, as those skilled in the art will plainly recognize. Additionally, polyvinyl alcohol can be etherified to produce kindred derivatized PVA's. A process for the production of such PVA derivatives is provided in U.S. Pat. No. 4,458,052. Such structurally similar derivatives are within the scope of the present invention.

Ser. No. 944,736, filed on Dec. 22, 1986, the inventor of which is a co-inventor of the present invention, discloses a process for the preparation of the copolymers of polyvinyl alcohol of equation (2), and is hereby incorporated by reference for all that it discloses. In that copolymerization process, PVA is copolymerized with acrylamide or other alpha, beta-unsaturated carbonyl compounds through the use of a low temperature (<100° C.) reaction which produces a high molecular weight modified PVA which is thermally stable. That one-step process comprises: forming a water slurry at room temperature of a PVA having a high degree of hydrolysis, a modifying agent and a base, and heating that mixture to a temperature of <100° C. for a time sufficient to obtain the resultant high viscosity, high molecular weight copolymeric product exemplified by equation (2). The resultant products are cold water soluble, contrary to the high molecular weight parent PVA, especially when the degree of hydrolysis is high (>90%). They are also more brine tolerant than the unmodified PVA and are thermally and hydrolytically stable in concentrated brines.

The crosslinking agent useful in the practice of this invention is a mixture of a phenolic component and an aldehyde. Phenolic components suitable for use in the present invention are phenol or derivatives thereof, such as catechol, resorcinol, phloroglucinol, pyrogallol, 4,4'-diphenol, and 1,3-dihydroxynaphthalene. Other phenolic components that can be used include at least one member of selected oxidized phenolic materials of natural or synthetic origin, such as 1,4-benzoquinone, hydroquinone or quinhydrone, as well as a natural or modified tannin, such as quebracho or sulfomethylated quebracho possessing a degree of sulfomethylation (DSM) up to about 50. (See U.S. Pat. No. 3,344,063, Col. 3, lines 15-32, which is hereby incorporated herein by reference). The DSM of sulfomethylated quebracho (SMQ) is sometimes indicated as, for example, SMQ 50 for SMQ having a DSM of 50. Phenol is the preferred phenolic compound for use in the present invention.

Any suitable water-dispersible aldehyde can be used in the practice of the invention. Thus, under proper conditions of use, both aliphatic and aromatic monoaldehydes, and also dialdehydes, can be used. The aliphatic monoaldehydes containing from one to about 10 carbon atoms per molecule are preferred. Representative examples of such aldehydes include formaldehyde, paraformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, valeraldehyde, heptaldehyde and decanal. Representative examples of dialdehydes include glyoxal, glutaraldehyde and terephthaldehyde. Various mixtures of such aldehydes can also be used in the practice of the invention. The term "water-dispersible" is employed generically herein to include both, those aldehydes which are truly water-soluble and those aldehydes of limited water solubility but which are dispersible in water or other aqueous media to be effective gelling agents. Formaldehyde is the preferred aldehyde compound used in the present invention.

Specific examples of suitable phenolic and water-dispersible aldehyde components are set forth in Swanson, U.S. Pat. No. 4,440,228, the entire contents of which are incorporated herein by reference. The preferred combination of a phenolic and water-dispersible aldehyde is phenol/formaldehyde.

Relative amounts of phenolic and aldehyde components are also set forth in Swanson. These amounts are small but effective to cause the gelation of an aqueous dispersion of the copolymer and the crosslinking agent. Thus, the amount of the phenolic component used herein is about 2 to about 20, preferably about 2 to about 10, and most preferably about 3 to about 6 percent by weight. Rigid and more thermally stable gels are generally formed at higher phenol concentrations. Aldehyde to phenol molar ratios of between about 1 to 4 are preferred. As such, the amount of aldehyde is about 0.002 to about 15 weight percent, preferably about 3 to about 10 and most preferably about 3 to about 7 weight percent. The above amounts are calculated on the basis of the total weight of the composition comprising the water, the copolymer and the crosslinking agent.

The amount of polyvinyl alcohol copolymer required to create the gel-forming compositions of the present invention will generally be an amount effective to form a stable gel in the presence of an organic crosslinking agent. Thus, the copolymers are generally used at concentrations ranging from 0.5 to 10.0 weight percent in order to achieve the desired gel consistency; in most cases, however, concentrations of 0.5 to 2.0 weight percent will be adequate and normally preferred, although reservoir conditions may require other concentrations. Generally, stronger and more stable gels are formed at higher copolymer concentrations. The molecular weight of the copolymer used will also affect the amount of material required to form advantageous gels. Generally, the higher the molecular weight of a polyvinyl alcohol copolymer, the lower the amount required to form a stable gel. It is preferred that the copolymer have a molecular weight of at least 10,000, with a molecular weight of at least 100,000 particularly preferred.

The gel-forming compositions of the invention can be prepared on the surface, in a suitable tank equipped with suitable mixing means, and then pumped down the well and into the formation employing conventional equipment for pumping such compositions. Additionally, it is within the scope of the invention to prepare these compositions while they are being pumped down the well. For example, a solution of the polymer in water can be prepared in a tank adjacent to the wellhead. Pumping of this solution through a conduit to the wellhead can then be started. Then, downstream from the tank, a suitable connection can be provided for introducing a cross-linking agent of this invention. As will be understood by those skilled in the art, the rate of introduction of the crosslinking agents into the conduit will depend upon the pumping rate of the copolymer solution through the conduit. Any of the above-mentioned orders of addition can be employed in such an in situ technique. Mixing orifices or baffles can be provided in the conduit, if desired. When gelation is to take place at lower temperatures (<300° F.), base catalysts such as NaOH, Ba(OH)₂, Ca(OH)₂ , Na₂ CO₃ or the like may be required to assist the reaction.

After the composition is injected into the formation and gels, the oil recovery process is conducted in the usual manner, i.e., a displacing fluid, which is miscible or immiscible with the oil, is injected into the formation and the oil is subsequently recovered in a conventional manner. Suitable oil-miscible displacing fluids are carbon dioxide (CO₂), methane, ethane, propane, butane, natural gas, liquid petroleum gas and mixtures thereof. CO₂ is the preferred oil-miscible displacing fluid. Suitable oil-immiscible displacing fluids are carbon dioxide, used under oil-immiscible conditions, water or an aqueous fluid, nitrogen, ambient air, steam, flue gas, and mixtures thereof. Because the gel-forming composition of the present invention is substantially stable at high temperatures (above about 400° F.), and does not undergo significant syneresis at such temperatures, it is preferably used in conjunction with the steam flooding of underground oil formations.

The gel-forming compositions of the present invention may also be used in a so-called Water Alternating Gas (WAG) process, well known to those skilled in the art. In such a process, the injection of slugs of water is alternated with the injection of slugs of gas, such as CO₂. If the WAG process is used with the compositions of the invention, such composition or compositions are injected into the formation with one or more water slugs.

After the miscible transition zone is established between the formation oil and the displacing fluid, a driving fluid may be injected through the injection well to displace the oil, the transition zone and the displacing fluid through the formation towards the production well from which the oil is produced. The driving fluid is injected for a sufficient time to effect the displacement of the formation oil to the production well until either all of the oil has been displaced from the formation or until the economical limit of the ratio of the driving fluid to the formation oil has been reached.

The driving fluid (also referred to herein as a drive fluid) used in the process of the invention may be any fluid known to those skilled in the art as suitable for that purpose, but preferably it is a fluid selected from the group consisting of water, brine, methane, carbon dioxide, nitrogen, air, steam, separator gas, natural gas, flue gas and mixtures thereof. The driving fluid may contain additives, such as a surfactant, to maintain efficient displacement thereof.

It is within the scope of the invention to precede the injection of the gelled composition into the well and out into the formation with a preflush of a suitable cooling fluid, e.g., water. Such fluids cool the well tubing and formation and may extend the useful operating temperature range of the gelled composition. The volume of the cooling fluid injected into the well can be any suitable volume sufficient to significantly decrease the temperature of the formation being treated, and can vary depending upon the characteristics of the formation. For example, at the wellbore, amounts of up to 20,000 gallons or more can be used to obtain a temperature decrease on the order of 100° to 250° F.

The copolymers of this invention are dissolved in water, at concentrations of about 0.5 to about 10.0 weight percent, preferably about 0.5 to about 2.0 weight percent. The solution is then injected into the formation, where it is to selectively block the more highly permeable regions of the reservoir, to control the subsequent flooding operation which may be carried out in a conventional manner. Injection of the solution into the formation may be carried out in a conventional manner using an injection well which extends from the surface of the earth into the formation, e.g. as described in U.S. Pat. Nos. 4,078,607, 3,305,016, 4,076,074, 4,009,755 and 4,069,869, to which reference is made for the descriptions of typical procedures which can be used herein. Briefly, the gel-forming composition is injected into the formation through the injection well and, in the formation, it is allowed to enter the more highly permeable stratum or strata in preference to the less permeable regions. The selective placement of the composition may be accomplished by utilizing the inherent temperature selectivity properties of the composition of this invention, or by adding a viscosifying material to the gel-forming composition to impart viscous selectivity, as those skilled in the art will readily understand. Such viscosifying agents include biopolymers such as xanthan and welan gums.

Once in place in the more highly permeable regions, the gel controls subsequent flooding operations by diverting the flood liquid, such as water or CO₂, to the less permeable or "tight" zones, increasing recovery from these zones. The amount of the gel-forming solution which is injected into the reservoir will generally be from about 10% to about 100% of the pore volume of the high permeability stratum or strata.

Because the compositions of the crosslinked copolymers may be readily varied, e.g., by changing the amount of the crosslinking agent or the degree of PVA hydrolysis, the viscosifying effects of the polymers may also be varied. This invention therefore offers the possibility of formulating polymers according to specific reservoir conditions.

The following examples further illustrate the essential features of the invention. However, it will be apparent to those skilled in the art that the specific conditions used in the examples do not limit the scope of the invention.

EXAMPLES 1-9

A poly (vinyl alcohol-vinyl propane sulfonate ether) copolymer [P(VA-VPSE)] was obtained in the manner previously disclosed and utilized to form the compositions detailed in Table 1. The particular P(VA-VPSE) utilized had a molecular weight of 60,000 and contained 3-4 mole percent of vinyl- propanesulfonate ether co-monomer units. The compositions of Examples 1-9 were prepared in a synthetic sea water brine. Thermal stability of those compositions were determined by subjecting small samples thereof (about 10 gr) to a temperature of about 400° F. for an extended time period. The samples, in small glass vials, were inserted into stainless steel bombs (1" O.D. tubing with Swagelock caps), partially surrounded with water, sealed, placed in an oven, and maintained at the temperature of about 400° F. for the length of time indicted in Table 1. After the testing period, the degree of syneresis of each sample was determined by cooling the bomb to ambient temperature, removing the vial from the bomb, estimating the size of the polymer plug by measuring the height and diameter of the gel plug, and calculating the degree of syneresis (%) from the following equation: ##EQU1## wherein:

"Sample Volume" is the initial volume of the sample before the heating treatment is commenced; and

"Gel Volume" is the final volume of the gel present in the vial after the heating treatment is terminated.

Results of the thermal stability tests are as follows:

                  TABLE 1                                                          ______________________________________                                         P(VA-VPSE) Phenol-Formaldehyde Gel Test                                                                            Gel                                        Example                                                                               Composition, %       Days    Syneresis                                  Number Polymer  Phenol  HCHO  NaOH  400° F.                                                                       %                                    ______________________________________                                         1      2.5      8       8.4   0.0   10    <5                                   2      2.5      8       8.4   0.4   10    10                                   3      2.5      8       8.4   0.8   10    <5                                   4      5.0      8       8.4   0.0   10    <5                                   5      5.0      8       8.4   0.4   10    10                                   6      5.0      8       8.4   0.8   10    10                                   7      7.5      8       8.4   0.0   10    10                                   8      7.5      8       8.4   0.4   14    20                                   9      7.5      8       8.4   0.8   14    20                                   ______________________________________                                    

As indicated, gels of excellent thermal stability were obtained in all cases.

EXAMPLES 10-16

A poly (vinyl alcohol-co-vinyl acrylamido ether) copolymer [P(VA-VAE)] was obtained using the copolymerization process previously described. The resultant P(VA-VAE) had a molecular weight of 133,000 and 8 mole % of vinyl acrylamido ether comonomer units. Like Examples 1-9, the compositions of Examples 10-16 were prepared in a synthetic sea water brine, having the constituents detailed in Table 2, below. Thermal stability was again determined in the manner described above. The results of these tests are as follows:

                  TABLE 2                                                          ______________________________________                                         P(VA-VAE) Phenol-Formaldehyde Gel Test                                                                             Gel                                        Example                                                                               Composition, %       Days    Syneresis                                  Number Polymer  Phenol  HCHO  NaOH  400° F.                                                                       %                                    ______________________________________                                         10     0.5      8       8.4   0.04  14    10                                   11     2.5      8       8.4   0.04  7     <5                                   12     2.5      8       8.4   0.00  7     5                                    13     2.5      4       4.2   0.02  7     <5                                   14     2.5      4       5.1   0.02  7     <5                                   15     2.5      3       3.0   0.02  7     10                                   16     2.5      3       3.9   0.02  7     <5                                   ______________________________________                                    

As may be seen from these test results, all gels were found to be very stable and would be expected to perform well even under the harsh conditions encountered during steam flooding enhanced oil recovery operations.

Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims. 

What is claimed is:
 1. A process for recovering oil from a subterranean oil-bearing formation having relatively high permeability zones and relatively low permeability zones penetrated by at least one production well in fluid communication with a substantial portion of the formation, comprising the steps of:(a) injecting into the formation an aqueous gel-forming composition comprising water, a water-dispersible polyvinyl alcohol copolymer, said polyvinyl alcohol copolymer selected from the group consisting of copolymers of vinyl alcohol and vinyl alkyl sulfonate ether having the structure: ##STR3## and copolymers of vinyl alcohol and vinyl-acrylamido ether, having the following structure: ##STR4## wherein: R is an alkyl or aryl group having up to 10 carbon atoms; M⁺ is Na⁺, K⁺ or other counter ions; and a and b are mole fractions of each co-monomer unit such that a+b=1, and a>o and b>o;and a crosslinking agent which is a mixture of a phenolic component and an aldehyde, said crosslinking agent present in an amount effective to cause gelation of said polyvinyl copolymer; (b) injecting a flooding fluid into the formation that preferentially enters the low permeability zones; and (c) recovering fluids including oil from the formation via the production well.
 2. The process of claim 1, wherein in step (a) said said polyvinyl alcohol copolymer has a molecular weight of at least about 10,000 and the comonomer which is not polyvinyl alcohol is present in an amount of about 1 to 30 mole percent.
 3. The process of claim 2, wherein in step (a) said polyvinyl alcohol copolymer has a molecular weight of at least about 50,000 and the comonomer which is not polyvinyl alcohol is present in an amount of about 1 to 10 mole percent.
 4. The process of claim 2, wherein in step (a) said polyvinyl alcohol copolymer is selected from the group consisting of poly (vinyl alcohol-co-vinyl-propanesulfonate ether) copolymer and poly (vinyl alcohol-co-vinylacrylamido ether) copolymer.
 5. The process of claim 4, wherein in said step (a) said phenolic mixture comprises about 1 to 99 weight percent of at least one phenolic compound selected from the group consisting of phenol, resorcinol, catechol, phloroglucinol, pyrogallol, 4,4'-diphenol and 1,3-dihydroxynaphthaline; and about 1 to about 99 weight percent of at least one aldehyde component selected from the group consisting of aliphatic monoaldehydes, aromatic monoaldehydes, aliphatic dialdehydes and aromatic dialdehydes.
 6. The process of claim 5, wherein in step (a) said phenolic compound is phenol, said aldehyde component is formaldehyde and said phenol and said formaldehyde are present in a molar ratio of about 1 to
 4. 7. The process of claim 6, wherein in step (a) said polyvinyl copolymer is present in an amount of between about 2 and 10 weight percent.
 8. The process of claim 1, wherein the composition used in step (a) further comprises a base catalyst to allow said gel-forming composition to gel at temperatures below 300° F.
 9. The process of claim 1, wherein the flooding fluid of step (b) is steam. 